1. Field of the Invention
The present invention relates generally to a rotary drive device and an image forming apparatus including the same.
2. Description of the Related Art
Conventionally, a rotary drive device configured to reduce variation in rotation speed of a drive motor, which transmits its rotational drive force to a driven rotor via a drive transmission portion such as a gear, by controlling a drive signal to be fed to the drive motor based on a rotation speed of the driven rotor detected using an encoder, which is a rotation information detector, is known.
For example, Japanese Laid-open Patent Application No. 2013-253680 discloses a rotary drive device, in which a disk-type encoder is attached to a driven gear on a driven shaft of a driven rotor. The driven gear is brought into mesh with a drive gear on a drive shaft of a drive motor. A rotation speed of the drive motor is feedback-controlled based on rotation information about the driven gear detected using a sensor arranged at a radially outer side of the encoder, thereby causing the driven rotor to achieve a target rotation speed.
The rotary drive device disclosed in Japanese Laid-open Patent Application No. 2013-253680 has, on a drive transmission path transmitting the rotational drive force of the drive motor to the driven rotor, drive transmission points including a mesh between the drive gear and the driven gear, a coupling between the driven gear and a parallel pin, and an insertion connection where the parallel pin is fitted in the driven shaft to be rotatable jointly with the driven shaft. At these points, periodic transmission variations occur, causing a plurality of resonance points to appear in frequency response characteristics derived from the rotation information obtained from the sensor. It is generally desired that feedback control be performed at a frequency sufficiently lower than a lowest one of resonance frequencies at the plurality of resonance points to avoid influence of resonance at the lowest resonance frequency. However, to increase the degree of freedom in design of a feedback control system, it is generally desirable to widen the control range in the feedback control.
In the rotary drive device disclosed in Japanese Laid-open Patent Application No. 2013-253680, first torsional stiffness at a mesh between the drive gear and the driven gear is set to be higher than second torsional stiffness at a coupling between the driven gear and the parallel pin. With this rotary drive device, a gain (i.e., amplification factor (the ratio of output amplitude to input amplitude in the encoder)) of resonance at a resonance point of a lowest resonance frequency in frequency response characteristics obtained when the drive motor is driven is lower than a gain of a configuration where the first torsional stiffness is set to be equal to the second torsional stiffness and a gain of a configuration where the first torsional stiffness is set to be lower than the second torsional stiffness. As a result, because influence of resonance at the resonance point can be reduced, even if the frequency of the feedback control for the drive motor is set close to the lowest resonance frequency, the feedback control can be performed favorably. Thus, according to Patent Document 1, because the feedback control frequency can be set close to the lowest resonance frequency, a control range of the feedback control can be widened.
If the lowest resonance frequency in the frequency response characteristics can be increased, the frequency (i.e., the feedback control frequency), at which periodic variation in rotation speed of the drive motor is sampled, can be set high. As a result, the control range, which is the range of applicable frequencies, of the feedback control can be widened. The technique disclosed in Japanese Laid-open Patent Application No. 2013-25368 can lower the gain at the resonance point of the lowest resonance frequency and reduce the influence of resonance at the resonance point, thereby bringing the feedback control frequency close to the lowest resonance frequency and, accordingly, can increase the feedback control frequency. However, because the lowest resonance frequency remains the same, the control range of the feedback control for the drive motor is not widened sufficiently.
It is an object of the present invention to at least partially solve the problem in the conventional technology.